Use AND when your research topic has more than one key term/idea that must be present in all of your search results. In this example, the key points are "soft drinks" and "childhood obesity." Searching with AND gives you fewer results because you're telling the database that you will only accept results that include both of your search terms.
Use OR when your research topic has synonyms which different authors may use interchangeably, for example "soft drinks" and "soda." This way, you won't miss out on potentially important search results because a paper or book was written with different wording than you are searching under. Searching with OR gives you more results, because you are telling the database that you're willing to accept any result that includes at least one of your search terms.
NOT when your research topic is often closely associated with another concept you do not want information about, for example if you are researching soft drinks and childhood obesity, but do not want information about vending machines in schools. Searching with NOT gives you fewer results because you're telling the database that you will only accept results that include one of your search terms by itself.
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